BOSTON (AP) — A mystery of sorts threatens to stunt Massachusetts’ small but growing oyster industry after illnesses linked to bacterial contamination forced the state to shut down beds for the first time ever.

The culprit is the Vibrio parahaemolyticus bacterium, which has occurred in state waters since the 1960s. Theories abound about the recent increase in illnesses linked to Massachusetts — but those are only theories.

“Honestly, I’m confused by the whole thing,” said Don Merry, an oyster grower from Duxbury, where oyster beds have been closed.

Average monthly daytime water temperatures in the region rarely approach the 81 degrees believed to be the threshold that triggers dangerous Vibrio growth. Rising average water temperatures locally, while not reaching that threshold, could be causing environmental changes that cause strains of Vibrio to thrive, said Suzanne Condon, associate commissioner of the Department of Health.

In addition, virulent Vibrio strains that aren’t as temperature-sensitive may have been carried from overseas in ships’ ballast water in the past decade, said the state’s chief shellfish biologist, Michael Hickey.

Meanwhile, it has been only six years since states were required to federally report Vibrio illnesses. So testing for it is relatively slow and underdeveloped and can’t yet predict, for instance, if outbreaks are coming, Hickey said.

The bacterium causes gastrointestinal problems, including vomiting and cramping, but the illness is generally severe only in people with weakened immune systems. The Centers for Disease Control and Prevention estimates the U.S. has about 4,500 cases of Vibrio infection annually.

Since May, Massachusetts has had 50 laboratory-confirmed cases of Vibrio, compared with 27 during the same period last year. Other states have also seen increases in Vibrio-related illnesses.

Last month, Connecticut closed oyster beds and issued a voluntary oyster recall after its first Vibrio outbreak, which sickened at least 14 people. Also last month, officials in Washington’s King County, where Seattle is located, warned oyster fans that Vibrio had sickened twice as many people as normal.

Cape Cod oyster farmer John Lowell said the trouble hits everyone working to build his state’s industry, though his East Dennis farm is nowhere near the closed beds.

“You either hang separately or you hang together, so it affects all of us,” he said.

Massachusetts has about 260 oyster growers who harvested roughly $12 million worth of oysters in 2012. That total is dwarfed by Louisiana — the highest-revenue oyster state, at $42 million — but it’s in the top five nationally, according to federal statistics.

Hickey said a boom in aquaculture operations fueled a 67 percent increase in the value of the Massachusetts oyster catch between 2010 and 2012.

Massachusetts’ first closures were announced Aug. 30 for oyster beds along the shore south of Boston, after illnesses caused by Vibrio were linked to an oyster-growing area in Duxbury. The second closure, announced Monday, shut down oyster beds in Katama Bay in Edgartown, on Martha’s Vineyard.

Sal Bartolomeo, an oyster eater from Boston’s North End, said that he’s always cautious with shellfish, and news of the closures will make him more so. Now, he’ll be sure to ask restaurants where they get their oysters, though he added he’ll likely just avoid oysters altogether for a while.

“But I wouldn’t give them up, like, for forever,” Bartolomeo said.

Vibrio growth is not pollution-related. Since it grows quickly at higher temperatures, state oyster dealers are under are strict handling requirements to keep the oysters cool, and it’s unknown where things went wrong with Massachusetts’ recent cases.

As researchers try to figure it out, they may also be able to find correlations in existing data that could help avoid future problems, Condon said. “Is there some way we can predict, maybe, when (harvesters) shouldn’t be collecting oysters?” she said.

Hickey figures it may be a month before the closed beds can reopen. Growers like Merry won’t lose their oysters during that time, just the chance to sell them.

No estimate of total lost sales is available. Merry said it’s costing him about $6,000 to $7,000 a week. Another cost, he said, may be higher.

“Quite honestly, the worst thing, is when we get back and rolling, is anybody going to want to eat a Duxbury oyster?” Merry said. “It’s hard to quantify how much this has hurt us.”